Broadly tunable optical couplers are important components for a number of applications such as wavelength division multiplexed networks and switching systems. Other applications which can benefit from tunable wavelength selective couplers are noise filters for optical amplifiers and intercavity wavelength selective couplers for monolithic extended cavity lasers. In addition, further benefits can result if the monolithic wavelength selective coupler can be integrated with a photodetector, optical amplifier, laser and the like.
Currently, most monolithic tunable lasers, such as the distributed feedback (DFB) or distributed Bragg reflector (DBR) devices, are limited to a tuning range of about 10 nm when using current injection which produces the greatest electrically induced index change. This limitation of about 10 nm results from the fact that the fractional tuning range, .DELTA..lambda./.lambda. of the fine-pitch Bragg grating wavelength-selective reflector is proportional to the normalized effective index change .DELTA.N/N.
In comparison, guided-wave bandpass transmission filters based upon forward coupling between asynchronous waveguide modes exhibit wavelength tuning at rates substantially greater than .DELTA.N/N. In a publication which appeared in Appl. Phys. Letters, 51, pp. 164-166, 1987 by F. Heismann et al., an electro-optically tunable filter based on periodic coupled mode conversion in lithium niobate waveguides was used as an inter-cavity filter in an early hybrid extended cavity laser to demonstrate broad laser tuning with a modest index change.
In a publication in Applied Phys. Letters, 59, pp. 2573-2575, 1991 by R. C. Alferness, it was reported that a grating-assisted vertical co-directional coupler can provide a bandpass filter and, in Tech. Digest, Integrated Photonics Research Meeting, Paper PDP8, Monterey, Calif., Apr. 9-11, 1991, R. C. Alferness et al. reported that this type of bandpass filter can be tuned over a range which is substantially wider than a grating reflector for the same induced refractive index change.
Recently, in electron. Letts., 27, pp. 2207-2209, Nov. 21, 1991, S. Illek, et al. reported on a tunable guide laser that includes a vertical coupler structure between two nearly synchronous guides. While evidence for extended tuning was observed, the tuning was unexpectedly irregular including, for example, sign reversal of the wavelength change.
Clearly, a monolithic implementation of a broadly tunable wavelength selective coupler which can be integrated with other devices, for example, an extended cavity gain means to form a tunable laser which is reliable in operation is needed.